In recent years, there has been an increase in the number of food products that are designed or intended to go directly from the freezer to the microwave for heating or cooking. Such food products often require packaging that also can go directly from freezer storage temperatures, typically temperatures ranging from about −20° C. to about 0° C., to microwave cooking temperatures, typically temperatures ranging from about 71° C. to about 105° C., without fracturing in the freezer or melting or exploding in the microwave. Typically, food manufacturers utilize rigid containers coupled with a lid for packaging frozen microwaveable foods. Rigid microwaveable containers are disclosed, for example, in U.S. Pat. No. 6,066,375; U.S. Pat. No. 6,379,497; and U.S. Pat. No. 4,836,438.
There exists in the art a variety of multilayer films that can be used as packaging for frozen food products. Such multilayer films are primarily made from polyethylene resins which have good cold-temperature resistance, i.e., being able to withstand temperatures ranging from about −20° C. to about 0° C. without shattering or fracturing. Polyethylene films, however, soften at microwave temperatures, typically temperatures ranging from about 160° F. to about 220° F. (about 71° C. to about 105° C.), which causes weakening of the bonding strength of the polyethylene seal layer and/or between the polyethylene film layers and adjacent layers, leading to seal failure and/or layer delamination and leakage of the package contents during microwaving. Thus, frozen food products must be removed from such food packaging materials and then placed on a microwaveable dish or container before the products can be heated or cooked in a microwave.
Microwaveable film packaging materials are also known in the art. One such packaging material is a 3-layer laminated film structure of polypropylene homopolymer/adhesive/polyethylene terephthalate (PET). However, this film packaging material becomes brittle when subjected to freezer temperatures and breaks apart or shatters. Thus, although the packaging material may be microwaveable, it is not suitable for frozen food applications.
To meet the requirements for freezable/microwaveable food packaging, the packaging materials must have two essential properties: (1) they must have good low temperature (subzero) resistance so that the packaging can withstand subzero freezer storage temperatures without fracturing or shattering; and (2) they must have good high-temperature resistance to prevent the package from melting or exploding in the microwave. Currently available film packaging materials can only handle one end of the temperature spectrum or the other, and therefore are not suitable for both storing frozen food products and then heating or cooking the food products in the microwave.
There is, therefore, a need in the art for a film packaging material that is designed to package frozen and refrigerated foods for subsequent direct microwave heating or cooking.